One of the most important kind of substance for the successful growing of roses is organic matter. Organic matter is matter that is derived from the breakdown of plant and animal residues. Composts, animal manures, shredded leaves, plant clippings, and shredded bark are common sources of organic matter. Besides containing variable amounts of plant nutrients, they also improve soil texture, air and moisture penetration, and moisture retention. Tight, fine textured soils like clay and clay loam, become looser, softer, and more penetrable by water -when organics are added, while loose soils, such as sandy soils, absorb and hold water more effectively when organics are added. Organic surface mulches – like pine needles, shredded bark, and wood chips, keep soils cooler in summer, suppress weed growth and reduce water loss.
When we first started growing roses 20 or 30 years ago, the most recommended organic soil additive was Canadian Peat moss. It is still a good one, but it is no longer inexpensive. We still use a little to mix with garden soil, for potting purposes, but we rely mostly on home grown mulch materials, such as lawn clippings, shredded leaves, and shredded rose canes, and tree trimmings for general soil improvement. We have a power shredder to shred our canes and leaves, but you can use a power lawn mower to reduce piles of leaves to a finer texture that can be piled as compost, or sacked for later incorporation in your garden soil.
The best time to incorporate organic matter for roses is when you build a new rose bed. Dig the bed deep and refill the hole with alternate layers of organic mulch material and good garden soil. When you dig the hole for planting the new roses, that will usually do a good enough job of mixing the soil and organic layers.
The next best time to add organics to the rose bed, is when you dig up a planted rose, and prepare to replace it with a new plant. Sometimes, after the bed is fully planted, I have taken my post hole digger and dug out holes 1 to 2 feet deep, between the planted roses. Then I fill those holes with organic mulch, or an organic rich soil mix. That gets organic matter into the root zone, without disturbing many of the rose roots.
Extra mulch or compost material can also be top-dressed on the surface of the rose bed. I sometimes do this with unsightly or only partly decomposed materials, and then cover it almost immediately with my summer mulch of tree bark. This improves sight and the smell.
Most mulch materials contain a lot of cellulose (wood). This is rich in energy, but low in nitrogen. When the soil bacteria break down the mulch, they find a rich supply of energy, but not much nitrogen. As a result they take from the natural nitrogen supply in the soil to produce their own cellular protoplasm. In so doing, they compete with our roses for nitrogen. This is one reason gardeners are often warned to mulch lightly so as not to deplete the available nitrogen. I prefer the advantages of a heavy mulch. A handful of nitrogen rich mineral fertilizer, or a little soluble fertilizer will easily replace the nitrogen temporarily depleted by the decay bacteria. .